1. Field of the Invention
The invention relates to an audio system comprising a circuit for processing an audio signal, whereby the circuit comprises an input for receiving the audio signal and an output for supplying an output signal, a harmonics generator coupled to the input for generating harmonics of the audio signal, and adding means coupled to the input as well as to the harmonics generator for supplying a sum of the audio signal and the generated harmonics to the output.
The invention further relates to a circuit for processing an audio signal, a harmonics generator and a method for processing an audio signal.
2. Description of the Related Art
An audio system according to the preamble is known from European Patent Application No. EP-A 546 619. Since the invention of the electrodynamic loudspeaker, there has been a need for greater acoustical output, especially at low frequencies. Often, however, for instance, in television sets or portable audio sets, this acoustical output is severely limited by the small size of the loudspeakers. It is known that this dilemma can be solved by using a psychoacoustic phenomenon often referred to as virtual pitch or missing fundamental, which evokes the illusion of a higher bass-response, while the loudspeaker does not radiate more power at those low frequencies. This illusion can be created by replacing low-frequency tones, which are present in the audio signal but cannot be reproduced by a small loudspeaker, by harmonics of these tones. The harmonics now represent the low-frequency tones.
In the known audio system, a low-frequency band of an audio signal is selected and supplied to a harmonics generator for generating harmonics of the selected signal. The generated harmonics are thereafter added to the audio signal. In this way, the low-frequency perception of the audio signal is improved. In the known audio system, a full-wave rectifier is used as the harmonics generator, which generates only even harmonics. A drawback of the full-wave rectifier is that the amplitude of the generated harmonics decreases rapidly with the number of the harmonic, e.g., with respect to the second harmonic, the amplitudes of the fourth, sixth and eighth harmonics are, respectively, 14 dB, 21 dB and 26 dB lower. Because of this reduction in amplitude of the generated harmonics, the virtual pitch effect cannot be fully exploited in the known audio system.